The following hypothesis is to be tested: a variety of substances and substrates, of which asbestos happens to be the best known, will excite phagocytes to produce and release reactive oxygen products. These reactive oxygen products may in turn attack precarcinogens and transform them into ultimate carcinogens. This mechanism is postulated to be independent from and distinct from the traditional monooxygenases dependent pathways for metabolic activation. It is expected that this chemical, extramicrosomal activation mechanism will afford a unifying theory to integrate disparate phenomena such as the "carcinogenicity" of fibrous minerals, particulates, some co-carcinogens, as well as the phenomena of solid state carcinogenesis, scar carcinoma, and carcinoma in hollow organs vis-a-vis foreign bodies (e.g., S. hematobium).